A wide variety of electronic circuit applications employ one or more voltage/current reference stages to generate precision voltages/currents for delivery to one or more loads/circuits. As a non-limiting example, equipments employed by telecommunication service providers typically contain what are known as subscriber line interface circuits (SLICs), that interface (transmit and receive) telecommunication signals with respect to (tip and ring leads of) a metallic (e.g., copper) wireline pair. To accommodate parameter variations in a telecommunication signalling environment, the SLIC is typically configured as a transconductance amplifier-based circuit, and may contain electrical parameter references (voltages/currents), whose values must be precisely maintained, irrespective of the voltages of the supply rails from which the SLIC is powered.
To this end it is common practice to employ a precision voltage reference element, such as a bandgap voltage reference device, from which a programmable output current or voltage may be derived. The basic operation of a bandgap device is to establish a voltage across a diode-connected transistor that is biased by a current which is proportional to temperature, and couple this temperature-proportional current through a resistor that is connected in series with the transistor.
A reduced complexity circuit architecture of a bandgap reference-based current mirror for producing a precision output voltage (and thereby ostensibly precision output current) is diagrammatically illustrated in FIG. 1. As shown therein a pair of bipolar NPN transistors QN and Q1 have their bases connected in common and to a bandgap voltage output node 11. In the circuit of FIG. 1, transistor Q1 serves as a bandgap junction device, whose emitter current I1 is proportional to temperature and flows to a current summation node 12. Transistor Q1 has its base-emitter junction voltage VbeQ1 coupled across companion transistor QN and resistor R1, with its emitter Q1e coupled to current summation node 12. Current summation node 12 is coupled through a resistor R2 to ground.
The bandgap voltage output node 11 is coupled through an output resistor R5 to a reference voltage terminal (here ground (GND)). In a typical integrated circuit layout, transistors QN and Q1 are located adjacent to one another and differ only in terms of the geometries by their respective emitter areas by a ratio of N:1. In the circuit of FIG. 1, the temperature proportional current I1 is definable as [(kT/q)lnN]/R1, where k is Boltzman's constant, q is the electron charge, T is temperature (in degrees Kelvin), N is the ratio of the emitter areas of transistors QN/Q1, and R1 is the resistance of resistor R1.
The collector QNc of transistor QN is coupled through the emitter-collector path of an NPN shielding transistor Q8 to the collector Q3c of a PNP transistor Q3 of a current mirror differential pair of PNP transistors Q3/Q4 having identical (1:1) geometries. The base Q8b of shielding transistor Q8 is coupled to the collector Q1c of transistor Q1. Transistor Q8 “shields” Early voltage effects on the current flowing through the collector terminal QNc of transistor QN. The emitter Q3e of transistor Q3 is coupled to a voltage supply rail VCC through a resistor R3, while the emitter Q4e of transistor Q4 is coupled to voltage supply rail VCC through resistor R4. Supply voltage rail-coupling resistors R3 and R4 have substantially identical resistance values and are used to minimize Early voltage effects on the collector current of transistor Q4.
Current mirror transistors Q3/Q4 have their bases Q3b/Q4b coupled to the emitter Q0e of PNP transistor Q0, the base Q0b of which is coupled to the collectors Q3c/Q8c of transistors Q3/Q8, and the emitter Q0e of which is grounded. The collector Q4c of current mirror transistor Q4 is coupled to the collector Q1c of transistor Q1, to the base Q8b of transistor Q8 and to the base Q6b of an output NPN transistor Q6. Output transistor Q6 has its emitter Q6e coupled to the bandgap voltage output node 11 and its collector Q6c to a bandgap referenced current drive output node 13. Output transistor Q6 performs the dual role of providing an output current port for the current flowing through resistor R5 and reducing base current errors in transistors QN and Q1 in the biasing of the bandgap transistors.
In the absence of parameter constraints, and ignoring potential base current errors, the circuit of FIG. 1 operates as follows. The bandgap transistor Q1 provides a prescribed forward bias diode voltage VbeQ1 to the series combination of the base-emitter junction QNbe of transistor QN and resistor R1, and across nodes 11 and 12. Due to the current mirror architecture within which transistors QN and Q1 are installed, a pair of identical emitter currents I1 are applied to the summation node 12 and thereby summed through the resistor R2 to ground. As a consequence, a precision bandgap voltage Vbg between node 11 and ground may be defined as: Vbg=VbeQ1+2I1(R2). Using this bandgap voltage structure, a bandgap-based current Ibg=Vbg/R5 may be supplied through output transistor Q6 to output current node 13. Thus, the circuit architecture of FIG. 1 may be used to source a precision bandgap-based voltage or a bandgap-based current. It should be noted that the current supplied through the output transistor Q6 contains two base current errors: 1—an error associated with the base currents of transistors Q1 and QN, and 2—an error associated with the base current of transistor Q6.
For present day silicon-based integrated circuits, and appropriate choice for the values of resistors R1 and R2, the bandgap-based output voltage Vbg is typically on the order of 1.2-1.25 volts. For a constrained supply rail voltage on the order of 3.0 volts, this leaves a difference or available overhead on the order of 1.8 to 1.75 volts to accommodate PN junction voltage drops (on the order of 0.6 volts each at room temperature) across the remaining series coupled transistors and voltage drops across the coupling resistors R3 and R4. While this difference may be tightly accommodated at room temperature, it is exceeded at the low temperature end (e.g., −40° C.) of the operational specification with which such circuits must comply, as PN junction voltages increase to values on the order of 0.8 volts.
In addition, as diagrammatically illustrated in FIG. 2, where the output current node 13 of the bandgap-reference circuit of FIG. 1 serves a current reference to an input mirror transistor Qm0 of a multiple (N) output port current mirror circuit 25, the contribution of base currents among the respective mirror transistors Qm1-Qmn becomes significant, mandating the use of a base current compensation or ‘beta-helper’ transistor Qh in the current supply path. The installation of a beta helper transistor brings with it an additional PN junction, that again causes the overhead voltage limit to be exceeded.